GENERAL HISTORY. 



The vitelline membranes are either simple structureless membranes 

 or present numerous radial pores. Membranes with the latter struc- 

 ture are very widely distributed, Echinodermata, Gephyrea, Vertebrata, 

 etc. (Vide figs. 5 and 7.) The function of the pores appears to be a 

 nutritive one. They either serve for the emission of pseudopodia-like 

 processes of the protoplasm of the ovum, as has beeu very beautifully 

 shewn in the case of Toxopneustes by Selenka (fig. 6), or they 

 admit (?) processes of the follicular epithelial cells (Vertebrata). 

 Their presence is in fact probably caused by the existence of such 

 processes, which prevent the continuous deposition of the mem- 

 brane. The term zona radiata will be applied to perforated mem- 

 branes of this kind. Two vitelline membranes, one perforated and the 

 other homogeneous, may coexist at the same time, e. g. Sipunculida, 

 Vertebrata. (Fig. 7.) 



The chorion is often ornamented with 

 various processes, etc. It is in many cases 

 doubtful whether a particular membrane is a 

 chorion or a vitelline membrane. 



All the membranes which surround the 

 ovum may be provided with a special aperture 

 known as the micropyle. A micropyle is by 

 no means found in the majority of types, and 

 there is no homology between the various 

 apertures so named. Micropyles have two 

 functions, either (1) to assist in the nutrition 

 of the ovum during its development, or (2) to 

 permit the entrance of the spermatozoa. The 

 two functions may in some cases coexist. 

 Micropyles of the first class are developed at 

 the point of attachment of the ovum to the 

 wall of the ovary or to its follicle. Good ex- 

 amples of this kind of micropyle are afforded 

 by the Lamellibranchiata (fig. 12), Holothuria, and many Annelida 

 (Polynoe, etc.). The micropyle of the Lamellibranchiata (p. 30) 

 probably serves also to admit the spermatozoa. The second type of 

 micropyle is found in many Insecta, Teleostei, etc. 



GENERAL BIBLIOGRAPHY OF THE OVUM. 



(1) Ed. van Beneden. " Recherche s sur la composition et la signification de 

 1'ceuf," etc. Mem. conr. d. I'Acad. ray. des Sciences de Belgique, Vol. xxxiv. 1870. 



(2) R. Leuckart. Artikel " Zeugung, " E. Wagner's Handworterbuch d. Physio- 

 logie, Vol. iv. 1853. 



(3) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Tkierwelt 

 xi. n. ihrer Bedeutung. " Oken. Isis, 1848. 



(4) Ludwig. "Ueber d. Eibildung im Thierreiche." Arbeit en a. d. zool.-zoot. 

 Institut Wiirzbttrij, Vol. i. 1874 J . 



(5) Allen Thomson. Article "Ovum" in Todd's Cyclopedia of Anatomy and 

 Physiolorjy, Vol. v. 1859. 



'(6) W. Waldeyer. Eierstock M. Ei. Leipzig, 1870. 



1 A very complete and critical account of the literature is contained in this paper. 



FIG. 7. SECTION 

 THROUGH A SMALL PART 

 OF THE SURFACE OF AN 

 OVUM OF AN IMMATURE 

 FEMALE OF SCYLLIUM 

 CANICULA. 



/ f . Follicular epi- 

 thelium, v t. Vitelline 

 Membrane. Zn. Zoua 

 radiata. yk. Yolk with 

 protoplasmic network. 



